Composite materials, or composites, are used in a wide variety of products. Composites are characterized by two or more distinguishable materials combined in a way that typically imparts a favorable strength to weight ratio. The constituents of composites include reinforcement materials and matrix materials. Reinforcement materials are usually stronger and more rigid than matrix materials, and provide strength and stiffness to the composite. Reinforcement materials can include particulates, whiskers (short fibers), or continuous fibers made from glass, metals, polymers, ceramics, or graphite, for example. The matrix material binds the reinforcement material and provides structure to the composite. Matrix materials include, for example, polymeric thermoset resins, such as polyester or epoxy resins, metals, or ceramics.
It is often desirable to impart spectral characteristics (e.g. color, gloss, reflectivity, absorption and/or emission of ultraviolet, visual, infrared, and/or radar frequency energy) to composite articles of manufacture. One method of imparting visual color is to simply paint a composite product, or to otherwise add a coloring layer to the composite. For example, U.S. Pat. No. 4,599,212 describes a resin useful as a gel-coat for composites that can be mixed with visual pigments to impart color. But gel-coat coloring composite products adds weight, manufacturing time, and cost to the products, and the coatings are often not as durable as may be required for many applications.
One approach to overcoming the weight and fragility problems of colored composites is set forth in U.S. Pat. No. 6,004,900, which recites a method including the step of forming a composite article using a resin mixture that includes a pigment. The pigment-containing resin hardens to form a pigmented matrix that has a durable color without adding significant weight to the composite. However, much of the pigment used in this method is unnecessary because it is distributed throughout the matrix, whereas only the outermost surface of the matrix is visible. This is undesirable because pigments are often costly, and so the amount of pigment used to impart desired spectral characteristics would preferably be minimized. Furthermore, pigment particles embedded throughout the matrix may compromise the strength of the composite.
Others have had some success applying only a layer of particles to the outer surface of composite materials. For example, in U.S. Pat. No. 5,091,234, there is a recitation of a method in which a layer of particles is deposited on an adhesive layer on a composite. However, these methods do not produce coatings that are sufficiently durable for many applications, and the adhesive layers can add significant additional weight and manufacturing cost to the final composite article.
There is thus a need in the art for a method of imparting spectral characteristics to composite products such that the treatment is durable and does not impart significant weight to the products, that need not use significantly more pigment than is necessary to impart one or more desired spectral characteristics, and that does not significantly increase the manufacturing cost of the product.